Carbon dioxide effects on the flammability characteristics of biogas

Flammability limits and flame speed of methane-carbon dioxide-air mixtures have been studied to understand the effect of carbondioxide on the flammability characteristic of biogas. The fuel of biogas discussed in this study was made by mixing gases of methane and carbon dioxide. The carbon dioxide was varied from 0% (by volume) untill reach the flammability limit of the stoikhiometri biogas-air mixtures. The observation was done using a cubic combustion bomb with the dimension of 500 mm × 200 mm × 10 mm with the initial condition being at room temperature and atmospheric pressure. The ignitor was set at the top of combustion bomb, so the flame propagated downward. Base on the observation results, the presence of carbon dioxide in the fuel ofbiogas caused the flammability limits of biogas-air mixture narrower. The biogas-air mixture was still flammable with the highest content of carbon dioxide of 62.5%vol when the mixture was sthoichiometri. Compared to methane-air mixture, the presence of carbon dioxide in biogas caused a reduction in the flame speed. The stoichiometri mixture has the highest flame speed when the carbon dioxide was not present in the fuel. However, when the carbon dioxide was added in the fuel, the rich mixture has the highest flame speed. This is a consequence of the rich biogas-air mixture having a higher fraction of the carbon dioxide components from the fuel compared to the stoichiometri and lean biogas-air mixture. The result also indicated that at the upper limit the flame still propagated downward to closed to the endwall. However, at the lower limit (lean mixtures), the flame did not intend to propagate downward, it was just at the top and propagate sideward.